Four-day school weeks have proliferated across the United States in recent years, reaching over 650 public school districts in 24 states as of 2019, but little is known about the effects of the four-day school week on high school students. This study uses district-level panel data from Oklahoma and a difference-in-differences research design to provide the first estimates of the causal effect of the four-day school week on high school students' ACT scores, attendance, and disciplinary incidents during school. Results indicate that four-day school weeks decrease per-pupil bullying incidents by approximately 31% and per-pupil fighting incidents by approximately 27%, but have no detectable effect on other incident types, ACT scores, or attendance.